I can understand that it will cause damage.
When you put something in the exhaust of a car the engine will block and turn off when it is full with gasses, after clearing the exhaust it will run again. And the pressure the engine will build is enough to blow a pato out of the exhaust.

For instance by a boat, there is probably a reason that the exhaust is above the water line when on speed.
With the FAE this is constant under water so at idle it can build up pressure because of the water.
When on speed it will suck the air and water and gasses out of your exhaust cause the venturi effect. This will not set any back presseru on the engine but I think it will suck on the engine to get more of it. Maybe this will cause the damage off that engine.